This invention relates generally to fire event sensing devices and, more particularly, to a system for more quickly sensing a critical temperature or a critical increase in temperature at any location within a monitoring area.
The use of various types of alarms for sensing fire event indicators such as heat or smoke are known. However, these devices include smoke or heat sensors disposed within a housing mounted on a wall or ceiling. An alarm is not sounded until sufficient amounts of smoke or heat reach the sensing devicexe2x80x94a time delay which may allow a fire to advance significantly and to decrease the available time for occupants of the dwelling to escape. The rapidity with which a fire advances makes fast detection and response to a fire event of crucial importance.
Various devices have been proposed for evaluating the electrical characteristics of wires or cables extending between multiple fire event sensors, such as the system disclosed in U.S. Pat. No. 3,821,734 to Herrliberg, et. al. This system, however, still uses individual sensors positioned in spaced apart locations. The device proposed in U.S. Pat. No. 4,361,799 to Lutz provides an over-temperature sensor including a positive temperature coefficient of resistance cable and capacitance elements to detect the presence of an over-temperature condition for the purpose of protecting mechanical equipment. While assumably effective for its intended purpose, the ""799 device does not provide the fastest response to the presence of a critical temperature at any location within a monitoring area and does not sense temperature or resistance rates of rise indicative of a fire event.
Therefore, it is desirable to have a fire event detection system that provides a fast response to a predetermined temperature occurring at any single cell location along a continuous path in a monitoring area. Further, it is desirable to have a system that energizes an alarm if a temperature rate of rise at any single cell location within the monitoring area exceeds a critical rate of rise parameter. It is also desirable to have a system which energizes an alarm if the resistance rate of rise at any single location is greater than the resistance rate of rise of the monitoring area as a whole.
A fire event detection system according to the present invention includes a programmable logic circuit capable of evaluating resistance input data supplied to it and comparing the evaluated data to various predetermined parameters. The system further includes a solid conductive positive temperature coefficient of resistance (PTCR) polymeric tape having opposed edges in contact with and enveloping respective conductors. The conductors have first and second ends electrically connected to the circuit. Therefore, the PTCR polymeric tape and conductors, also referred to collectively as xe2x80x9cthe sensorxe2x80x9d, extend in a generally U-shaped or fold-back configuration throughout a monitoring area such as a room of a house or an attic.
The edges of at least a portion of the sensor define a plurality of notches for repeatedly redirecting current transversely across the PTCR polymeric tape. Adjacent notches are spaced apart along opposed edges. In essence, this establishes a plurality of single sensing cells which sense resistance in series as opposed to sensing resistance in parallel as is the case with unnotched PTCR polymeric tape. Further, an increase in temperature at a particular cell has a greater effect on resistors configured in series than on resistors configured in parallel, making detection of a hot spot swifter using series resistors. The PTCR polymeric tape provides a sharp increase in resistance at a predetermined temperature. The conductors provide electrical resistance input data to the circuit for evaluation. The circuit energizes an alarm if the resistance data indicates a resistance greater than a critical resistance (i.e. the critical temperature has been exceeded). Further, the alarm is sounded if the evaluated resistance data indicates an average temperature rate of rise that is greater than a critical rate of rise. This condition indicates that a temperature increase in the monitoring area is not merely a natural climate change. Further, the circuit will energize the alarm if the resistance rate of rise of any single cell is greater than the resistance rate of rise of the unnotched portion of the PTCR polymeric tape. This indicates the existence of a hot spot, i.e. the temperature at one location is increasing faster than the temperature of the entire room or attic as a whole.
This invention is particularly well suited for use in an attic environment as the circuit evaluates whether increases in ambient air temperature are merely climate changes or whether the increases truly indicate the presence of a fire. However, the invention is also well-suited for use in traditional fire detection environments such as dwellings or businesses and provides a faster response to fire event indicators at any single cell location along the elongate sensor.
Therefore, a general object of this invention is to provide a fire event detection system that responds quickly to fire event indicators occurring at any single cell location in a monitoring area.
Another object of this invention is to provide a system, as aforesaid, which energizes an alarm if the temperature of a PTCR polymeric sensor exceeds a predetermined critical temperature.
Still another object of this invention is to provide a fire event detection system, as aforesaid, which energizes an alarm if the rate of rise of the average temperature of the sensor exceeds a critical rate of rise parameter.
Yet another object of this invention is to provide a fire event detection system, as aforesaid, which energizes an alarm if the temperature rate of rise of any single cell is greater than the rate of rise of the temperature of the unnotched portion.
A further object of this invention is to provide a fire event detection system, as aforesaid, which includes an elongate PTCR polymeric tape that extends throughout a monitoring area.
A still further object of this invention is to provide a fire event detection system, as aforesaid, in which the polymeric tape includes a plurality of resistance sensing cells configured in series rather than in parallel.
Another object of this invention is to provide a fire event detection system, as aforesaid, that is easy and economical to manufacture.
Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, an embodiment of this invention.